1. Field of the Invention
The present invention relates to a surface-coated member such as a surface-coated cutting tool that is coated with a hard coating layer having excellent chipping resistance and high wear resistance, and particularly to a surface-coated cutting tool that shows high breakage (fracture) resistance and high cutting performance under harsh cutting conditions.
2. Description of Related Art
A common type of cutting tool used widely in metal cutting operations is the surface-coated cutting tool that comprises a base body made of cemented carbide, cermet, ceramic or the like that is coated with one or more hard coating layer such as TiC layer, TiN layer, Al2O3 layer or TiCN layer formed on the surface thereof.
As high efficiency cutting operations become commonplace, conventional cutting tools experience such problems as the hard coating layer cannot endure strong impact generated in cutting operations where the cutting edge receives strong impact such as heavy intermittent cutting of metal, eventually resulting in chipping of the rake surface or peel-off of the hard coating layer. Thus service life of the cutting tool is limited by sudden occurrence of tool breakage such as breakage or abnormal wear of the cutting edge.
Japanese Patent No. 3230372 described that breakage resistance of a cutting tool can be improved by forming the hard coating layer where TiCN layer that includes columnar crystal is divided by a granular TiN layer or the like thereby to suppress peel-off of layers.
However, the constitution disclosed in Japanese Patent No. 3230372 cannot solve the problem that the hard coating layer is liable to peel-off near the interface between the Al2O3 layer and the TiCN layer. Thus cutting operations where the cutting edge receives strong impact such as heavy intermittent cutting of metal have still been prone to chipping and/or peel-off of the hard coating layer in the interface between the Al2O3 layer and the TiCN layer. In case thickness of the hard coating layer is decreased for the purpose of preventing chipping and peel-off of the hard coating layer, the hard coating layer disappears prematurely, resulting in accelerated wear and failure to extend tool life of the cutting tool.
In case attention is directed only to the adhesion force between the Al2O3 layer and the TiCN layer, adhesion force between the TiCN layer and the base body is compromised, thus leading to peel-off from the TiCN layer and failure to elongate the service life of the cutting tool.
Japanese Unexamined Patent Publication No. 2000-158205 discloses such a constitution as the proportions of carbon C and nitrogen N contents in the TiCN layer made of stringer-like TiCN crystal (longitudinally grown TiCN crystal) are varied, with the upper layer (Al2O3 layer) side made of TiCN having higher nitrogen content and the lower layer (base body) side made of TiCN having higher carbon content, so that occurrence of chipping is reduced during high-speed cutting operations.
However, in case castings such as gray cast iron (FC) or ductile cast iron (FCD), or steel having inhomogeneity in hardness or unusual shape is cut, sporadic application of strong impact to the cutting edge of the tool causes the coating layer including the TiCN layer to peel off, thus exposing the base body and rapid progress of wear. Moreover, when thickness of the layer involves variability among individual tools, thinner Al2O3 layer leads to plastic deformation due to lower wear resistance. Thicker Al2O3 layer, on the other hand, causes the coating layer including the TiCN layer to peel off, thus exposing the base body and resulting in rapid progress of wear. Such variability in the performance related to the film thickness has been conspicuous.
Japanese Patent No. 3269305 disclosed such a process that, after a titanium-based hard layer including a TiCN layer has been formed, heat treatment is carried out in hydrogen atmosphere of 10 to 100 torr at a temperature from 850 to 1100° C. for a duration of one to five hours, so that W and Co are diffused in the grain boundary of TiCN crystal, thereby to improve bonding between the titanium-based hard layer and the aluminum oxide layer of the hard coating layer.
With the constitution described in Japanese Patent No. 3269305, however, the cutting edge is still subject to abnormal wear due to chipping, thus resulting in short service life of the cutting tool under harsh cutting conditions which are often employed recently such as heavy intermittent cutting where the cutting edge is subject to sudden application of strong impact. In case thickness of the hard coating layer is decreased for the purpose of preventing chipping and or peel-off of the hard coating layer, the hard coating layer disappears prematurely, resulting in accelerated wear and failure to elongate the service life of the cutting tool. Also there have been demands for further improvements in breakage resistance and in wear resistance for cutting of steel and other materials.
Japanese Unexamined Patent Publication No. 11-269650 describes that bonding between the Al2O3 layer and the TiCN layer can be improved by interposing such a Ti2O3 layer having a mean thickness of 0.1 to 2 μm between the Al2O3 layer and the TiCN layer that shows X-ray diffraction pattern having maximum diffraction peak at a diffraction angle (2θ) of 24.0±1 degrees observed in X-ray diffraction analysis using Cu-k α line as the beam source. However, titanium oxide cannot endure the high-load machining operations which are dominant recently.
Japanese Unexamined Patent Publication No. 9-174304 describes such a constitution as an intermediate layer consisting of needle-like grains as viewed in the cross section is provided between a titanium carbonitride layer and the aluminum oxide layer formed on the surface of the former, so as to restrain the aluminum oxide layer from peeling off by anchoring effect and prevent the wear resistance from decreasing.
With the constitution of interposing the intermediate layer consisting of needle-like grains between the titanium carbonitride layer and the aluminum oxide layer, however, although peel-off of the aluminum oxide layer can be prevented, it has been necessary to further improve the breakage resistance of the hard coating layer.
Japanese Unexamined Patent Publication No. 10-109206 describes that crystal width of the Al2O3 layer side in a pillar-shaped crystal TiCN layer is increased 1.0 to 1.3 times of the crystal width of the base body side, thereby suppressing membrane separation from an interface with Al2o3 layer and TiCN layer, and preventing the tool damage such as abnormal wear and sudden breakage.
According to Japanese Unexamined Patent Publication No. 10-109206, however, although the tool damage by membrane separation can be prevented, the breakage-proof nature and wear resistances of a hard covering layer itself are insufficient, and the enough tool life cannot be acquired. Therefore, the further improvement of the breakage-proof nature and wear resistance of the hard covering layer was demanded.